1. Field of the Invention
The present invention relates to a method of recovering ammonia by the distillation of an aqueous solution containing ammonia, carbon dioxide, and hydrogen cyanide.
2. Description of the Prior Art
Aromatic nitrites are useful as the raw material for the production of synthetic resins, agricultural chemicals, etc. and the intermediate material for amines, isocyanates, etc. Heterocyclic nitrites are useful as the intermediate material for medicines, feed additives, food additives, etc. The reaction of a cyclic hydrocarbon or heterocyclic compound each having an organic substituent with ammonia in the presence of an oxygen-containing gas is called ammoxidation. The nitrile compounds are generally produced by a vapor-phase catalytic ammoxidation. It is well known in the art to conduct the ammoxidation in the presence of a catalyst containing vanadium, molybdenum, iron, etc. For example, JP 11-209332A discloses the production of nitrile compounds by the ammoxidation of alkyl-substituted cyclic hydrocarbons or heterocyclic compounds in the presence of a catalyst containing V—Cr—B—Mo oxide. JP 9-71561A discloses the production of dicyanobenzene by the ammoxidation of xylene in the presence of a catalyst containing Fe—Sb—V oxide.
To produce the aromatic nitrile and heterocyclic nitrile in high yields, ammonia is used in excess of its theoretical amount in these methods. In the ammoxidation of alkyl-substituted cyclic hydrocarbons or heterocyclic compounds, ammonia is used 1.5 to 10 times the molar amount of the alkyl group. To produce the aromatic nitrites or heterocyclic nitrites with low costs, therefore, it is required to recover the non-reacted ammonia from the gas which remains after separating the nitrile compounds from the reaction product gas and recycle the recovered ammonia to the reaction system.
To recover the non-reacted ammonia from the remaining gas after separating the nitrile compounds form the ammoxidation product gas and recycle the recovered ammonia to the reaction system, the non-reacted ammonia is absorbed by water and ammonia is recovered from the obtained solution by distillation in known methods.
For example, “Hydrocarbon Processing,” February 1976, pp. 103-106 describes the production of aromatic nitrile by the vapor-phase, catalytic oxidative ammonolysis in a fluidized bed manner in the presence of a catalyst containing a metal oxide. In the proposed method, the reaction product gas is cooled by a cooler to collect the nitrile compound. Thereafter, the remaining gas is introduced to an absorber where the non-reacted ammonia and the by-produced hydrogen cyanide are dissolved in water to separate from the waste gas (carbon monoxide, nitrogen, etc.). The resultant aqueous solution is introduced to a stripper where it is separated into a waste water containing high-boiling substances and a distillate containing ammonia, hydrogen cyanide, water, etc. The distillate was fed to a distillation tower where ammonia and an aqueous solution containing hydrogen cyanide and ammonium carbonate are separated from each other. The aqueous solution was separated into hydrogen cyanide/carbon dioxide gas and ammonia/water. The ammonia/water is returned to the distillation tower at the preceding stage.
“Chemical Engineering,” November 1971, pp. 53-55 discloses a process of producing isophthalonitrile. In the proposed process, the ammoxidation of m-xylene with ammonia and air is performed in a fixed bed reactor in the presence of a catalyst containing vanadium oxide. The reaction product gas is introduced to a scrubber, where the gas is cooled to crystallize isophthalonitrile. The remaining gas is introduced to an absorber where the non-reacted ammonia and the by-produced hydrogen cyanide are dissolved in water. Waste gas from the top of absorber is sent to incineration. It is described that the ammonia recovered from the aqueous solution from the scrubber and absorber is recycled to the reaction system. However, the document is completely silent about the details thereof.
The inventors have proposed in JP 2001-348370A a method of producing a nitrile compound by the reaction between a cyclic hydrocarbon or heterocyclic compound each having an organic substituent, ammonia and an oxygen-containing gas. In the proposed method, the nitrile compound is separated from the reaction product gas and the non-reacted ammonia and by-produced hydrogen cyanide in the gas remaining thereafter are absorbed by water. The obtained aqueous solution is distilled under a pressure of 0.2 to 0.7 MPa to recover ammonia and hydrogen cyanide, which are then returned to the reaction system.
None of the above documents describe the corrosive properties of an aqueous solution containing ammonia, carbon dioxide and hydrogen cyanide nor the materials for the portion of a distillation apparatus which comes into contact with the aqueous solution.
“Table of Corrosion Resistance of Material for Chemical Apparatus,” revised and enlarged edition, Kagaku Kogyo Sha, Co., Ltd. 1984, pp. 83, 88, 91 describes the corrosive properties of an aqueous solution of ammonium carbonate (ammonia-carbon dioxide-water mixture), ammonia water, and hydrogen cyanide, respectively. In this document, these corrosive properties are rated as Corrosion Degree A, namely, the corrosive properties against which a general purpose stainless steel such as SUS304 (austenitic stainless steel known as 18-8 stainless) can sufficiently resist.